The present invention relates to rotary drum mixers, e.g., parallel flow, counterflow and concentric flow mixers, of the type for mixing, heating and/or drying particles, for example, aggregate used in the asphalt industry for surfacing roads, and particularly relates to apparatus and methods for controlling the exhaust gas temperature of the drum mixer to a predetermined temperature. The invention may also be used in some kilns.
It is conventional in many industries to use generally horizontally disposed rotating drums to dry a wide variety of solid particles. Typically, a burner generates hot combustion gases and the gases flow through the drum while it is rotating to dry the particles in the drum. The burners may be fueled by gas, oil or coal. Flighting is frequently employed in the drum to facilitate the heat transfer between the hot gases of combustion and the particles. Particularly, the flighting picks up the particles from the bottom of the drum and, as the drum rotates, permits the particles to fall or cascade in the drum to create a veiling effect. Typically, the veiling pattern is such that the particles are distributed substantially across the entirety of the width of the drum. In certain applications, for example, as set forth in U.S. Pat. No. 4,189,300 of common assignee herewith, the flighting is specifically designed to distribute the particles in a predetermined pattern across the drum and for particularly preventing the particles from veiling in a certain area of the drum. The purpose of preventing the veiling action in that patent, however, is to preclude the cascading particles from interfering with the flame of the burner.
In typical rotary drum mixing and drying systems, such as used in the asphalt industry, the rotary drum mixer forms only a part of an asphalt plant which also includes hoppers for aggregate supplies, silos for storing the hot mix (as described below), a baghouse for cleaning the exhaust gases, and other ancillary equipment, such as conveyors, fuel preheaters, etc. It is frequently important in such plants to maintain the exhaust gas temperature from the drum within predetermined limits. However, various operating parameters often determine the exhaust gas temperature. For example, in the asphalt industry, the product mix between different sized aggregates is often varied. Additionally, recycled asphalt materials are frequently utilized, either by themselves or for mixing with virgin aggregate. Additional asphalt is also provided the asphaltic composition to obtain the proper product mix for surfacing roads. Additionally, in both of the currently conventional parallel and counterflow asphalt mixing plants, the veiling action of the flights changes because of the varying quantities of material passing through the virgin portion of the dryer and the gradation of the material. Further, the moisture content of the aggregate affects the heat transfer rate between it and the hot gases of combustion. With all of these parameters in mind, it has been very difficult to control the exhaust gas temperature without degrading efficiency and driving up costs. Nonetheless, it is important to control the exhaust gas temperature of those gases exiting to the baghouse so that the exhaust gas temperature is above the dewpoint temperature but below a safe operating level for the exhaust system. Such operating level may typically be about 400.degree. F.
Methods for controlling the exhaust gas temperatures have previously included varying the slope of the drum, i.e., the inclination of the axis of rotation of the drum, and the rotary speed of the drum. In addition, the flights inside the drum may be changed to create greater or lesser veiling action and hence determine, to a limited extent, the exhaust gas temperature for a given aggregate gradation and mix. An additional burner can also be placed at the dryer gas outlet and used to maintain the temperature above the dewpoint. Each of these methods, however, has drawbacks. For example, significant downtime and hence costs are incurred should the flights be changed. Often the "fix" is limited to a single product mix, necessitating similar costly changes for other product mixes. Other inefficiencies creep into the system when these methods are used to control the exhaust gas temperature.
In accordance with the present invention, there is provided novel and unique apparatus and methods for controlling the exhaust gas temperature of a rotary drum mixer. Particularly, the veiling of the particles is adjusted, without changing or replacing flighting, to create a channel in the particle veil such that a portion of the hot gases bypasses the cascading particles. In this manner, the average outlet gas temperature is increased because the flow of hot gases in the channel is not in heat transfer relation with the veiling particles in the drum. That is to say, by diverting or intercepting at least a part of the veil of particles within the drum to define a channel substantially free of particles, a portion of the hot gases bypasses the particle veil and flows through the channel, hence increasing the average temperature of the exhaust gas in comparison with the temperature of the exhaust gas without diverting or intercepting the particle veil.
Preferably, the flighting is arranged to provide a substantially even veil of particles across the interior of the drum without holes or channels for the exhaust gas to bypass the cascading particles. Thus, the hot gases passing through the cascading veiled particles are in heat transfer relation with the particles. Consequently, the exhaust gas temperature is lowered resulting from the transfer of heat to the particles. By intercepting or diverting part of the veiled particles to create a hole, void or channel through the veil, the present invention enables a portion of the hot gases of combustion to exit the drum, either without passing through the particle veil in heat transfer relation with the veiled particles or passing through the particle veil only to a limited extent. Consequently, the average exhaust gas temperature will be higher than would otherwise be the case if none of the veiling particles were diverted or intercepted.
In a preferred embodiment of the present invention, by appropriate design of the flighting and other parameters, a very heavy particle veil is provided in the dryer. In this manner, exhaust gas temperatures substantially lower than the desired exhaust gas temperature for the exhaust gas system can be created with such heavy veil design, resulting in exhaust gas temperatures below the dewpoint temperature. This would precipitate water and dust in the exhaust gas system, causing substantial problems. The present invention enables, however, a very heavy veil design, with the greater efficiencies afforded thereby, while simultaneously enabling the exhaust gas temperature to be controlled to the desired temperature above the dewpoint. To accomplish that, the present invention provides a blade or an obstruction in the interior of the drum which is adjustable to intercept or divert a greater or lesser volume of the cascading or veiling particles to define a hole or channel in the veiling particles and hence reduce the transfer of heat from the hot combustion gases to the veiling particles. For example, a blade is mounted on a control shaft such that, upon rotation of the shaft, the blade intercepts a greater or lesser extent of the veiling particles, creating a channel or hole in the particle veil below the blade, enabling hot gases of combustion to flow directly through to the exhaust without heat transfer to the veiling particles. The blade may be rotated about an axis generally parallel to the axis of rotation of the drum or may be moved in a generally axial direction of the drum, or both, to vary the volume of the hole or channel, thereby regulating the temperature of the exhaust gases.
Another significant aspect of the present invention resides in a feedback system for controlling the magnitude of the intercepted or diverted veiled particles dependent upon the temperature extant in the exhaust. Thus, a temperature sensor is provided in the exhaust and a controller, responsive to the detected temperature of the exhaust gas, manipulates the blade to divert or intercept veiling particles to a greater or lesser extent, depending on the difference between the extant and desired exhaust gas temperatures. If, for example, the exhaust gas temperature is too low, the blade or obstruction is diverted to intercept additional veiling particles to enlarge the dimensions or volume of the channel thus formed, enabling a greater proportion of the hot gases to flow through the drum without engaging in heat transfer relation with the particles. If the exhaust gas temperature is too high, the blade or obstruction is diverted to intercept fewer veiling particles, enabling a greater proportion of the hot gases of combustion to be placed in heat transfer relation with the veiling particles flowing through the drum.
As indicated above, the present invention is particularly useful in asphalt plants. It improves the overall efficiency of the system by providing an optimum exhaust gas temperature above the dewpoint temperature yet below a temperature where safety may be jeopardized. It also creates the lowest possible actual gas volume through the exhaust system, which aids in overall exhaust gas system efficiency.
In a preferred embodiment according to the present invention, there is provided apparatus for mixing, heating and drying solid particles comprising a rotatable drum having an inlet for supplying particles into the drum and an outlet for discharging the mixed, heated and dried particles from the drum, means for supplying hot gases of combustion for flow along the interior of the drum to heat the particles in the drum and an exhaust outlet for the hot gases within the drum. Flighting is provided within the drum for creating a veil of particles in the interior of the drum in response to rotation of the drum and through which veil hot gases of combustion flow in heat transfer relation with the particles, together with means within the drum for intercepting at least a part of the veil of particles within the drum to define a channel substantially free of particles such that a portion of the hot gases flowing along the drum bypasses the particle veil and passes through the channel.
In a further preferred embodiment according to the present invention, there is provided apparatus for mixing, heating and drying solid particles comprising a drum rotatable about a generally longitudinal axis and having an inlet for supplying particles to the drum and an outlet for discharging the mixed, heated and dried particles, means for supplying hot gases of combustion for flow along the drum to heat the particles in the drum and means for displacing the particles along the drum between the inlet and the outlet. An exhaust outlet is provided for the hot gases of combustion within the drum. Flighting is disposed within the drum between the inlet and the outlet and is responsive to rotation of the drum for creating a veil of particles within the drum in heat exchange relation with the hot gases flowing along the drum. Means are also provided within the drum for forming a channel through the particle veil substantially free of particles such that a portion of the hot gases flowing along the drum bypasses the particle veil and passes through the channel.
In a still further preferred embodiment according to the present invention, there is provided apparatus for mixing, drying and heating solid particles comprising a rotatable drum having an inlet for supplying particles to the drum and an outlet for discharging the mixed, dried and heated particles, means for supplying a stream of hot gases of combustion within the drum in heat transfer relation to the particles in the drum, an exhaust outlet for the hot gases within the drum, flighting within the drum for creating a veil of particles within the interior of the drum in response to rotation of the drum and through which veil hot gases of combustion flow in heat transfer relation with the particles and means for variably controlling the proportion of hot gases flowing in the drum in heat exchange relation with the particle veil.
In a still further preferred embodiment according to the present invention, and in an an apparatus for mixing, heating and drying solid particles in a rotatable drum having flighting within the drum for creating a veil of particles in the interior of the drum and through which veil hot gases of combustion flow in heat transfer relation with the particles, a method of controlling the exhaust gas temperature from the drum comprising the step of variably controlling the proportion of hot gases flowing in the drum in heat exchange relation with the particle veil by intercepting at least a part of the veil of particles within the drum to define a channel substantially free of particles such that a portion of the hot gases flowing along the drum bypasses the particle veil and passes through the channel.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for controlling the exhaust gas temperature of a rotary drum mixer by adjustment of the particle veiling and, hence, the heat transfer relation between the hot gases of combustion and the particles.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.